This invention relates to a method of manufacturing a shaped article of hydrophilic hydrogel insoluble in water. The shaped articles of this hydrogel are mainly used as medical materials.
In recent years, the application of high molecular weight materials to the medical field has become a subject of active study. Among other high molecular weight materials, hydrogels which are insoluble in water and yet hydrophilic have come to attract increasing attention because of their permeability to various substances (gases, ions, molecules, etc.) and their ability to contain water. In fact, they have now come to be used as various separation membranes, catheters, soft contact lenses, blood preserving containers, cell cultivating substrates, blood circulating paths, and embedding substrates such as for enzymes and medicines.
For such purposes as mentioned above, hydrogels which are based on such compounds as alkylene glycol mono(meth)acrylates and (meth)acrylic acids have been preponderantly used. Numerous methods have been proposed for the production of such hydrogels. Particularly as methods for manufacturing shaped articles of hydrogels with high repeatability, for example U.S. Pat. No. 3,660,545 discloses a process for producing a contact lens by polymerizing a glycol ester of (meth)acrylic acid in a rotating mold, U.S. Pat. Nos. 4,121,896 and 4,108,364 disclose a process for producing a soft contact lens by polymerizing a monomer mixture in a two-piece mold while having a flexible rim component attached to either of the two pieces of said mold, U.S. Pat. No. 4,347,198 discloses a process for molding a contact lens by filling an original solution in a space between a concave and a convex mold and gelling said original solution.
For any method which prepares a shaped article of hydrogel within a mold, the most important thing is how to remove the cast product from the mold. Particularly when the casting is carried out by solution polymerization in a mold, the manner of the removal of the cast product poses itself an important problem because the gel in a state containing a solvent has low strength. Further when a shaped article is desired to be cast of a hydrogel containing a hydroxyl group or carboxyl group in the molecular unit thereof, the cast product in the process of being removed from the mold tends to sustain damage because the hydroxyl group or carboxyl group interacts strongly with the surface of the xold.
As a solution to this problem, U.S. Pat. No. 3,499,862 teaches a method which comprises polymerizing in a mold a monomer mixture consisting of a polyhydric alcohol ester of an .alpha.,.beta.-unsaturated monocarboxylic acid, 0.1 to 2.0% by-weight, based on the weight of the polyhydric alcohol ester of the .alpha.,.beta.-unsaturated monocarboxylic acid, of a crosslinking agent proper for the ester, and at least 40% by weight, based on the weight of the mixture, of an inactive liquid miscible with water, contacting to water the shaped article of hydrogel produced by the polymerization to shrink in the mold after the polymerization, and removing the shrunk shaped article from the mold. In the specification of this U.S. patent, however, there is a statement to the effect that if the amount of the inactive liquid is less than 40%, the shaped article of hydrogel is swollen at at the detriment of the optical property thereof when this article is washed with water so that this method produces the shaped article of hydrogel in a size invariably smaller than the size of the mold.